I want to use the value of CL and/or Cd as criteria for convergence. I'm thinking of calculating something like (CL(i)-CL(i-1))/CL(i-1), where i is the current iteration and i-1 is the previous. How can i get the values of variables from the previous and current iteration and do such a calculation?

It is not recommended to converge based on a global parameter like this. It is entirely possible that the lift coefficient could stabilize for a while then change. For instance, the boundary layer may continue to develop slowly and have little effect on lift, then separate, which will have a dramatic effect on lift. In such a case, your convergence criteria would completely miss the separation.

The best criteria for convergence are the MAX and RMS residuals. It would be better to determine what MAX or RMS residual level is needed to get your lift coefficient within a required tolerance and stick with this value.

Keep in mind that simply observing that a value doesn't change does not mean you have a solution.